Blew it up... GRRRRR!!!!!
I had the same problem with stock rocker arms. I believe they are good for low lift but anything over .450 they don't hold up very well (just guessing).
But for facts I had a .420 cam in my engine and ran it hard with no problems. Then I installed a .474 cam and one intake and one exhaust punched through on each head. I didn't have coil bind or anything like that because I checked before starting the engine and with my engine builder friend. I went with a cheap brand of roller rockers for a while then switched to Comp rollers.